Heat treatment for improving proof stress of nickel-chromium-cobalt alloys



United States Patent O HEAT TREATMENT FOR IMPROVING PROOF STRESS OF NICKEL-CHROMIUM-COBALT AL- LOYS Carol Henry White, Burley Gate, and William Iain Mitchell, Solihull, England, assignors to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed Oct. 20, 1965, Ser. No. 499,087

8 Claims. (Cl. 148-162) The present invention relates to a process for heat treating nickel-chromium-cobalt alloys and, more particularly, to a process wherein certain precipitation hardenable, nickel-chromium-cobalt alloys are subjected to a special sequence of heat treating operations with the objective of enhancing the proof stress thereof such that the alloys can be more satisfactorily forged into large blanks for rotor discs.

As is well known to those skilled in the art, conventional and standard procedures for heat treating precipitation hardenable, nickel-chromium-cobalt alloys used for elevated temperature applications include solution heating the alloys, cooling the same and thereafter applying either a single or double aging treatment followed by cooling. The solution heating step is normally carried out at comparatively high temperatures, the aging treatment being conducted at somewhat lower temperatures. Where a double aging treatment is utilized, the first aging step is usually performed at a temperature above that of the second. Such treatments, quite naturally, are designed to bring forth the optimum in mechanical characteristics of the alloys, including high stress-rupture values, good ductility, adequate proof stress, etc.

Alloys of the type above-mentioned are set forth in our US. Patent No. 3,151,981 wherein it is explained that nickel-chromium-cobalt-base alloys, precipitation hardened by titanium and aluminum and containing columbium and molybdenum, must be solution heated and thereafter aged, e.g., in the temperature range of 600 C. to 850 C., in order to develop their full mechanical properties, including their proof stress at elevated temperatures up to about 600 C. To obtain the highest level of proof stress, a double aging treatment is recommended, to wit, heating for from two to four hours at 750 C. to 850 C., air cooling, and heating for from 16 to 40 hours at 680 C. to 720 C., e.g., at 700 C. However, even when so treated the proof stress of the alloys is lower than is desirable for some uses thereof, notably in respect of large forged blanks for rotor discs. To increase the proof stress, say, by two or three long tons per square would render the alloys quite satisfactory with regard to proof stress for the above-mentioned application.

It has now been discovered that the proof stress of the subject alloys at temperatures up to 600 C. can be improved by the application of a special heat treatment which includes a pre-aging step immediately after a solution heating step, provided that the alloys are of a special composition as described herein.

It is an object of the present invention to provide a process for heat treating certain herein-described precipitation hardenable, nickel-chromium-cobalt alloys whereby the proof stress of the alloys is sufficiently enhanced to render them suitable for large forged blanks for rotor discs.

Other objects and advantages will become apparent from the following description.

Generally speaking, thepresent invention comprises subjecting special nickel-chromium-cobalt alloys to the following sequence of heat treating operations: (1) solution heating for a period of about one-half hour to eight hours within the temperature range of about 900 C. to about 1200 C., (2) cooling to retain the solution formed, (3) pre-aging the alloys by heating them within the range of about 550 C. to about 700 C. for a period from about five minutes up to about 16 hours, (4) aging the alloys at a temperature above the pre-aging temperature, and

(5) thereafter cooling to room temperature.

The alloys processed in accordance herewith contain (in percent by weight) from 0.03% to 0.09% carbon, about 14% to 22% chromium, about 10% to 20% cobalt, about 4% to 5.7% columbium, up to 0.3% tantalum, the sum of the columbium plus one-half the tantalum (if any) being from 4% to 5.7%, about 4% to 6.5% molybdenum, from about 2% to 3.5% titanium, up to 0.8% aluminum, the combined content of titanium and aluminum being at least 2.5%, about 0.001% to 0.01% boron, and about 0.01% to 0.1% Zirconium, the balance, except for impurities and residual deoxidants, being nickel. The major impurities that may be present are silicon, manganese and iron in amounts up to 1% each. The total amount of impurities and residual deoxidants should not exceed 2% and preferably it is as low as is practicable. In particular, it is desirable to keep the silicon content below 0.3%.

With further regard to the compositional ranges set forth above, it is worthy of note to mention that the exact theory which might explain the apparent phenomenon involved is perhaps not completely understood. However, it can be said without much reservation that the heat treatment in accordance herewith does not significantly improve the proof stress of all nickel-chromium alloys. Actually, the use of a pre-aging step before either a single or double aging treatment is known in itself, having been used in order to improve the creep resisting properties of high temperature alloys which were columbium-free. The purpose of the step when so used was to bring about the rapid production of nuclei of a precipitable phase and thereafter the aging temperature was raised slowly to a higher temperature .at which the precipitated particles will grow more rapidly. The alloy was then held at this high temperature until the desired hardness was obtained. As used hitherto, this heat treatment, insofar as we are aware, has not conferred any significant increase in proof stress (the specific objective of the subject invention) but only improvement in creep resistance. However, the creep resistance of the alloys herein is quite satisfactory for intended purposes.

One of the factorsv which might very well exert a pronounced influence stems from the fact that the alloys herein are columbium-containing and this constituent is necessary to provide optimum proof stress in combination with other metallurgical properties. Now, it has been found that the columbium-containing alloys of the invention differ in their response to heat treatment from alloys devoid of columbium but otherwise similar in composition. In the columbium-free alloys the main hardenmg phases formed on aging are the gamma (gamma prime) phase, the composition of which is usually represented as Ni (Ti,Al), and carbides. In the columbiumcontaining alloys of the invention, however, the gamma phase or a structural. modification thereof of the same composition known as gamma (gamma double prime), is stable only at fairly low temperatures up to about 700 C. On prolonged heating above 700 C., the gamma phase is transformed into the eta phase, Ni (Ti,Cb). This phase forms as an intragranular precipitate, whereas in the columbium-free alloys such eta phase as does form is a cellular precipitate, a cellular precipitate which appears to form at the grain boundaries. The intragranular precipitate of eta phase is much less effective than the gamma phase in hardening the alloys and insuring high proof stress at elevated temperatures. In order to bring then reheated to the temperature of the next step, or if desired it may be transferred directly to a furnace at the next temperature. It should not be cooled more slowly than in air between the solution heating and pre-aging,

about aging in a commercially practical time, it is never- 5 since this may lead to the precipitation of eta phase. theless necessary to heat the alloys at temperatures at The invention is particularly useful with alloys of which eta phase is normally formed. Overcoming the rather more limited composition than that set forth heredilemma posed was achieved by finding a way of altering inbefore, namely, from 0.04% to 0.09% carbon, 18% or changing the nature of the aforementioned intragranto 22% chromium, 13% to 15% cobalt, 4% to 6.5% ular precipitate which affected tensile properties. Accordmolybdenum, 2.1% to 2.75% titanium, 0.3% to 0.8% 1O lngly, it has been found that if the sub ect alloys are aluminum, the combined titanium and alumlnum contents heated in the pre-aging step for long enough to form exceeding 2.5%, from 4.7% to 5.3% columbium, from nuclel of gramma phase, then on aging at a higher tem- 0.001% to 0.01% boron and from 0.01% to 0.1% zirperature all or most of the precipitate formed consists of conium, the balance, except for impurities, being nickel. gramma phase. 15 One such alloy was made by air melting and vacuum In carrying the invention into practice, the exact conrefining, that is to say, by heating in the molten state d1t1ons of time and temperature used for solution heating under vacuum. It was found on analysis to contain 0.05% are not critical provided incipient melting of the alloy carbon, 19.9% chromium, 2.36% titanium, 5.05% co- 1s avolded and this heating may be effected, as referred to lumbium, 0.55% aluminum, 14.3% cobalt, 4.47% molybherelnbefore, in the temperature range of 900 C. to denum, 0.0015% boron and 0.035% zirconium, the bal- 1200 C. for a period from about /2 hour to 8 hours. In ance being nickel and impurities. Ingots cast from the order to bring about substantially complete solution of alloy were converted to hot rolled bar /s inch in diamall precipitable inter-metallic phases, the temperature is eter. Portions of the bar were given a variety of heat preferably at least 1050 C., e.g., 1100 C. to 1150 C. treatments and test pieces machined from the heat treated At lower temperatures, only partial solution of those bar were subjected to tensile and creep tests at 575 C. phases occurs but this may be advantageous if it is de- The heat treatments consisted basically of solution heatsired to retain the strengthening effect of prior cold working for 1 hours at 1130 C.; water quenching; aging 111g. for 4 hours at 800 C.; air cooling; second stage aging In the pre-aging step, the temperature must not be so from 16 hours at 700 C.; and again air cooling. Behrgh that eta phase is nucleated instead of gamma On tween the solution heating and initial aging steps the the other hand, if it is too low, the nuclei of gamma phase bars (except test bars 9 and 10 in the table below) were take too long to form. It is for these reasons that the preheated at 650 C. for varying lengths of time followed aging is effected in the temperature range of 550 C. to by air cooling. The results are set out in the following 700 C. and the duration of the pre-aging should be from table:

TABLE Test Time at 0.1% proof U.T.S., EL, R.A., Stress, Total plastic bar 650 0., stress, t.s.i. t.s.i. Percent Percent t.s.i. strain, percent hours at 100 hrs.

1 54. 6 77.7 20. 0 25 43 0. 04 1 78.8 24.3 26 42.6 0.06 2 53. a 76. 7 25. 7 32 0. 05 2 54. 7 77. 9 20. 7 20 45 0.06 4 54. 9 77.8 23. e 20 45 0. 05 4 55. 5 77. 5 22. 9 27 45 0. 04 8 55. 2 76. s 17.1 25 45 0. 05 8 55. 1 7s. 6 23. 5 29 45 0. 0s 0 51. 6 75. 7 27. 9 30 45 0. 02 0 50. 0 72. 9 15. 0 21 45 0. 03

t.s .=Long tons per square inch.

R.A. =Reduction in area.

5 minutes to 16 hours, longer times being required at lower temperatures. Preferably, the temperature is in the range of 630 C. to 670 C. and the period of heating is from A hour to 8 hours, e.g., 4 hours, at 650 C. After pre-aging in this way, an extremely fine precipitate of gamma phase is just detectable on examination by the electron microscope using the thin foil technique.

The pre-aging step is followed by aging, the object of which is to grow a precipitate of gamma phase on the nuclei formed by pre-aging. The temperature must, therefore, be high enough for this growth to take place in a reasonable time but not so high that eta phase is precipitated. For these reasons the aging temperature is preferably from 750 C. to 850 C. While aging may be completed in one step at this temperature, it is preferable not to continue heating until the hardness and strength no longer increase, since by finishing the aging at a lower temperature, higher values of hardness and strength can be achieved. Thus, the alloys are advantageously aged by heating for from 2 to 5 hours in the range of 750 C. to 850 C., e.g., for 4 hours at 800 C., followed by a second stage of aging in the range of 680 C. to 720 C. for from 4 to 40 hours, e.g., at 700 C. for 16 hours.

After each step of the heat treatment the alloy may either be air cooled or quenched in oil or Water, and

The data in the above table reflects that the proof stress and also tensile strength of the alloy as pre-aged (test bars 1 through 8) were markedly higher than in the case where the alloy was not pre-aged (test bars 9 and 10). The average 0.1% proof stress was on the order of four long tons per square inch higher than for the conventionally treated specimens.

A further advantage of the invention is that the properties of the alloys are more consistent when the pre-aging step is applied, that is to say, the scatter is reduced.

The alloys and heat treatment of the invention are particularly useful for the production of rotor discs for gas turbine engines and of other articles and parts requiring a similar combination of properties.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be Within the purview and scope of the invention and appended claims.

We claim:

1. A process of heat treating a precipitation hardenable, colum ium-containing, i fil-chromium-cobalt alloy to boron, about 0.01% to 0.1% zirconium and the balance,"

except for impurities and residual deoxidants, being nickel, to a solution heating within the temperature range of 900 C. to 1200 C. for from about 5 minutes to 16 hours, cooling the alloy to retain the solution formed, developing a fine precipitate of gamma or gamma phase without the concomitant occurrence of any appreciable quantity of eta phase by pre-aging the alloy in the temperature range of 550 C. to 700 C., aging the alloy at a temperature above the pre-aging temperature, and thereafter cooling the alloy.

2. A process as set forth in claim 1 in which the solution heating is effected in the temperature range of 1050 C. to 1150 C.

3. A process as set forth in claim 1 in which the aging is effected in the temperature range of 750 C. to 850 C.

4. A process as set forth in claim 1 in which the preaging is effected within the temperature range of about 630 C. to 670 C. for about hour to 8 hours.

5. A process as set forth in claim 1 in which the aging is stopped before the hardness and strength no longer increase, and a second aging at a lower temperature is then effected.

6. A process as set forth in claim 5 in which the first aging is effected by heating for from 2 to 5 hours in the range of 750 C. to 850 C. and the second aging by 6 heating in the range of 680 C. to 720 C. for from 4 to hours.

7. A process of heat treating a precipitation hardenable, columbium-containing, nicklel-chromium-cobalt alloy to improve the proof stress characteristic thereof which comprises subjecting an alloy containing from 0.04% to 0.09% carbon, 18% to 22% chromium, 13% to 15% cobalt, from 4.7% to 5.3% columbiurn, 4% to 6.5% molybdenum, 2.1% to 2.75% titanium, 0.3% to 0.8% aluminum, the combined titanium and aluminum contents exceeding 2.5%, from 0.001% to 0.01% boron and from 0.01% to 0.1% zirconium, the balance, except for impurities, being nickel, to a solution heating within the range of 1050 C. to 1150 C. for from about hour to 8 hours, cooling the alloy to retain the solution formed, pre-aging the alloy over the temperature range of about 630 C. to 670 C. for a period of 4 hour to 8 hours to thereby develop a fine precipitate of substantially the gamma phase but without concomitantly forming any appreciable quantity of the eta phase, aging the alloy at a temperature of 750 C. to 850 C. and thereafter cool ing the alloy.

8. The process as set forth in claim 7 wherein the aging treatment is conducted for about 2 to 5 hours which in turn is followed by a second aging treatment for from 4 to 40 hours within the temperature range of 680 C. to about 720 C.

References Cited CHARLES N. LOVELL, Primary Examiner, 

7. A PROCESS OF HEAT TREATING A PRECIPATION HARDENABLE, COLUMBIUM-CONTAINING, NICKEL-CHROMIUM-COBALT ALLOY TO IMPROVE THE PROOF STRESS CHARACTERISTIC THEREOF WHICH COMPRISES SUBJECTING AN ALLOY CONTAINING FROM 0.04% TO 0.09% CARBON, 18% TO 22% CHROMIUM, 13% TO 15% COBALT, FROM 4.7% TO 5.3% COLUMBIUM, 4% TO 6.5% MOLYBDENUM, 2.1% TO 2.75% TITANIUM, 0.3% TO 0.8% ALUMINUM, THE COMBINED TITANIUM AND ALUMINUM CONTENTS EXCEEDING 2.5%, FROM 0.001% TO 0.01% BORON AND FROM 0.01% TO 0.1% ZIRCONIUM, THE BALANCE, EXCEPT FOR IMPURITIES, BEING NICKEL, TO A SOLUTION HEATING WITHIN THE RANGE OF 1050*C. TO 1150*C. FOR FROM ABOUT 1/2 HOUR TO 8 HOURS, COOLING THE ALLOY TO RETAIN THE SOLUTION FORMED, PRE-AGING THE ALLOY OVER THE TEMPERATURE RANGE OF ABOUT 630*C. TO 670*C. FOR A PERIOD OF 1/4 HOUR TO 8 HOURS TO THEREBY DEVELOP A FINE PRECIPITATE OF SUBSTANTIALLY THE GAMMA '' PHASE BUT WITHOUT CONCOMITANTLY FORMING ANY APPRECIABLE QUANTITY OF THE ETA PHASE, AGING THE ALLOY AT A TEMPERATURE OF 750*C. TO 850*C. AND THEREAFTER COOLING THE ALLOY. 